Star Citizen developer Cloud Imperium Games was at the PAX East convention last weekend, where it handed out codes that give players access to parts of the game for free for a limited time. Even if you weren't there, you can still enter a code to get the same access.
You'll have to create a free Star Citizen client on the game's website, then go to this page, and enter the code: PAXEASTFREEFLY2K15.
This will let you walk around the Hangar Module and sit in an Anvil Hornet F7C trainer ship that you can then take into flight in Arena Commander, a multiplayer and wave-based, combat oriented mode. Once activated the pass is valid until March 15, 2015.
Luckily, while the client for the full game looks like it will come in at around 100GB, the client for the demo is only a little over 20GB.
Star Citizen, already the most successful crowdfunding project of any kind in history, has now raised over $75 million.